Inkjet recording apparatuses generate very low noise, can print images at high speed, and can use a variety of inks and inexpensive plain paper. Because of these advantages, inkjet recording apparatuses are widely used as image recording apparatuses or image forming apparatuses such as a printer, a facsimile machine, and a copier. An inkjet recording apparatus or a liquid-droplet jetting apparatus includes a nozzle for jetting ink droplets; a liquid chamber including a discharge chamber connected to the nozzle, a pressurized liquid chamber, a pressure chamber, and an ink channel; and a pressure generating unit for causing ink in the liquid chamber to be discharged. Examples of pressure generating units include a piezoelectric-type pressure generating unit that uses an electromechanical transducer such as a piezoelectric element to deform or displace a vibrating plate forming a wall of the discharge chamber and thereby to cause ink to be discharged; and a bubble-type (thermal-type) pressure generating unit that uses an electrothermal converting element such as a heat element in the discharge chamber to generate air bubbles by film boiling of ink and thereby to cause the ink to be discharged. There are various types of piezoelectric-type pressure generating units such as a longitudinal vibration type employing deformation in the d33 direction; a transverse vibration (bend mode) type employing deformation in the d31 direction; and a shear mode type employing shear deformation. Also, with the advancement in semiconductor processes and microelectromechanical systems (MEMS), a thin-film actuator produced by forming a liquid chamber and a piezoelectric element directly on an Si substrate has been proposed.
An electromechanical transducer used for the piezoelectric-type pressure generating unit includes a lower electrode (first electrode), an electromechanical transduction layer, and an upper electrode (second electrode) that are stacked on each other. With this configuration, a piezoelectric element for generating pressure to discharge ink is provided for each pressure chamber. The electromechanical transduction layer is made of, for example, lead zirconate titanate (PZT) ceramic that includes multiple metal oxides as major components and is generally called a metal composite oxide.
In a related-art method of forming an electromechanical transducer for each pressure chamber, an electromechanical transduction layer is formed on a lower electrode using a known deposition method such as a vacuum deposition method (e.g., sputtering, metallo-organic compound chemical vapor deposition (MO-CVD), vacuum evaporation, or ion plating), a sol-gel method, a hydrothermal synthesis method, an aerosol deposition (AD) method, or a metal organic decomposition (MOD) method. Next, an upper electrode is formed on the electromechanical transduction layer. Then, the upper electrode, the electromechanical transduction layer, and the lower electrode are patterned by photolithography and etching. Here, it is not easy to process a metal composite oxide, particularly PZT, by dry etching. While an Si semiconductor device can be easily processed by reactive ion etching (RIE), a special RIE method, where ICP plasma, ECR plasma, and helicon plasma are used in combination to increase plasma energy of ionic species, is required to process a metal composite oxide. Such a method increases the costs of a production apparatus. Also, with this method, it is difficult to achieve a sufficient selection ratio with respect to an underlying electrode film. Particularly, with a large-area substrate, non-uniformity in the etching rate is critical. Although an etching step may be omitted by forming an etching-resistant PZT film only in desired areas, such a method has not been tried sufficiently.
For the above reasons, several methods for selectively forming a PZT film have been proposed.
One of the methods is a hydrothermal synthesis method where PZT is selectively grown on Ti metal. This method makes it possible to grow a PZT film only on a patterned Ti electrode. To obtain a PZT film with a sufficient pressure resistance by this method, the thickness of the PZT film is preferably 5 μm or greater. If the thickness is less than 5 μm, dielectric breakdown may easily occur when an electric field is applied to the PZT film.
Another one of the methods is a vacuum deposition method that is used, for example, to form a patterned luminescent layer using a shadow mask in the production of an organic EL display. With this method, a PZT film is formed at a substrate temperature of 500-600° C. The substrate temperature of 500-600° C. is necessary to crystallize the composite oxide and thereby to induce piezoelectricity.
Still another one of the methods is an aerosol deposition (AD) method where a resist pattern is formed in advance by photolithography and a PZT film is formed in areas without the resist film. Similar to the hydrothermal synthesis method, the AD method is suitable for forming a thick film. With this method, a PZT film is also deposited on the resist film. Therefore, a lift-off step is performed after removing a part of the deposited film by grinding.
Further, there is a liquid-droplet jetting method where a PZT precursor is applied at high resolution.
However, with the hydrothermal synthesis method, it is difficult to form a thin film with a desired thickness. Also, when forming a device on an Si substrate with the hydrothermal synthesis method, it is necessary to protect the Si substrate because the hydrothermal synthesis occurs in a strong alkaline aqueous solution. With the vacuum deposition method, because of a difference in the thermal expansion rate between the Si substrate and the shadow mask, which is normally made of stainless steel, it is difficult to effectively mask the Si substrate. Also, a disposable shadow mask is impractical. Particularly, a disposable shadow mask is not suitable for the MO-CVD method and the sputtering method where wraparound of a deposited film tends to easily occur. The AD method is not suitable for forming a thin film with a thickness less than 5 μm. Also, it is difficult to uniformly grind a large area. Further, since the resist film has a low heat resistance, it is necessary to perform aerosol deposition at the ambient temperature and to convert the deposited film to a film having piezoelectricity through a post-annealing process. With the liquid-droplet jetting method, since the amount of the PZT precursor liquid applied to a platinum surface is very small, the liquid tends to dry quickly. Also, at an edge of an application area where the vapor concentration of a solvent evaporated from the small amount of liquid is low, the liquid tends to dry quickly. The difference in the drying rate of the PZT precursor liquid between different areas of the platinum surface results in uneven thickness of an electromechanical transduction film which in turn degrades the electrical characteristics of an electromechanical transducer.